For example, as described in Japanese Patent Application Publication No. 2007-20348, in an inverter-controlled brushless DC motor, a minute voltage is induced between the output shaft and the frame of the motor. If this voltage is higher than the voltage allowed against the insulation resistance of bearing grease, electric discharge takes place on the bearing rolling surface, so that the bearing may be damaged by the electric discharge machining effect. This phenomenon is called electrolytic corrosion.
The occurrence of electrolytic corrosion produces flaking, pitting, fluting (washboard-form pattern), and the like on the bearing rolling surface, and these defects cause an early bearing failure.
Accordingly, in Japanese Patent Application Publication No. 2007-20348, bearing brackets provided at the right and left of a stator are electrically connected to each other via a conductive tape to decrease the potential difference between the output shaft and the frame, thereby preventing electrolytic corrosion from occurring.
Unfortunately, the conventional method for preventing electrolytic corrosion has posed problems described below. In Japanese Patent Application Publication No. 2007-20348, the brackets are electrically connected to each other via the conductive tape, and the conductive tape peels off easily because of being a tape. Also, the productivity is degraded by the increase of the process for sticking the conductive tape.
Furthermore, since the conductive tape is uncovered on the motor surface, care must be taken not to bring other parts into contact with the conductive tape when the motor is incorporated into a product. Also, the conductive tape may be peeled off by transportation or shock.